Secondary-side dynamic load detection and communication device

ABSTRACT

A switching power converter provides regulated voltage to a load. The switching power converter comprises a transformer including a primary winding coupled to an input voltage and a secondary winding coupled to an output of the switching power converter. The switching power converter further comprises a power switch coupled to the primary winding and a rectifier coupled to the secondary winding. Current is generated in the primary winding responsive to the power switch being turned on and not generated responsive to the power switch being turned off. A detection circuit measures a voltage across the rectifier. If the detection circuit detects a decrease in the voltage across the rectifier outside of a blanking period, the detection circuit generates a current pulse in the secondary winding of the transformer. A fast power-on-reset device generates a startup signal to activate the detection circuit at an end of the blanking period.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 14/340,482, filed Jul. 24, 2014, which is incorporated hereinby reference in its entirety

BACKGROUND

This disclosure relates generally to a switching power supply and moreparticularly to a switching power converter detecting dynamic loads viaa secondary-side detection device.

Many electronic devices, including smart phones, tablets, and portablecomputers, employ power supplies providing controlled and regulatedpower output over wide operating conditions. These power supplies ofteninclude a power stage for delivering electrical power from a powersource to a load across a transformer. A switch in the power stageelectrically couples or decouples the load to the power source, and aswitch controller coupled to the switch controls on-time and off-time ofthe switch. Regulation of the power output can be accomplished by, amongother things, measuring the output current or output voltage and feedingthat back to the primary-side switch controller. In order to improvecost performance and reduce size, many commercially available isolatedpower supplies employ primary-only feedback and control. By sensingprimary side signals, the secondary output and load condition can bedetected and thus be controlled and regulated.

For convenience, end users often leave the power supply connected to theAC mains at times when no load is connected to the power supply output.To maintain a regulated output voltage even in no-load conditions, thecontroller may change its regulation mode under low load or no-loadconditions. Under no-load conditions, the rate of the pulses that turnon or turn off the power switch of the switching power converter isdecreased significantly in order to maintain output voltage regulation,resulting in long periods of time between ON and OFF cycles of theswitching power converter. This presents a significant challenge toprimary-side sensing control schemes that rely on the ON and OFF cyclesof the power switch to obtain a feedback signal. During the periodsbetween ON cycles of the switch, the status of the output voltage isunknown by the controller as no feedback signal is generated. If theelectronic device is reconnected to the power supply, representing adynamic load change, during one of the long OFF cycles of the switch,the primary-side controller does not receive feedback about the changein the secondary side output voltage until the next ON cycle of theswitch. In the interim, the output voltage may therefore dropsignificantly, exceeding the allowable voltage drop specified by theregulation specifications of the switching power converter or theelectronic device.

SUMMARY

A switching power converter detects load transients at an output of theswitching power converter using a secondary-side detection device. Inone embodiment, the switching power converter comprises a transformerincluding a primary winding coupled to an input voltage and a secondarywinding coupled to an output voltage of the switching power converter. Apower switch is coupled to the primary winding of the transformer, and arectifier is coupled to the secondary winding of the transformer.Current is generated in the primary winding responsive to the powerswitch being turned on and is not generated responsive to the powerswitch being turned off The rectifier provides a rectified current tothe output of the switching power converter during off cycles of thepower switch.

A detection circuit, coupled across the rectifier on the secondary-sideof the transformer, measures a voltage across the rectifier. During offcycles of the power switch, the voltage across the rectifier isindicative of the output voltage of the switching power converter. Ifthe voltage across the rectifier falls below a threshold value outsideof a blanking period, the detection circuit detects the voltage dropand, in response, generates a current pulse in the secondary winding ofthe transformer. A fast power-on-reset (POR) device coupled to thedetection circuit generates a startup signal to activate the detectioncircuit at an end of the blanking period, enabling the detection circuitto detect the voltage drop.

In one embodiment, a switch controller detects the voltage change on theprimary side of the transformer as a result of the current pulse on thesecondary side of the transformer. The switch controller turns on thepower switch in response to detecting the pulse to regulate the outputvoltage of the switching power converter. The switching power convertertherefore compensates for dynamic load conditions during off cycles ofthe power switch based on the current pulse generated by the detectioncircuit.

The features and advantages described in the specification are not allinclusive and, in particular, many additional features and advantageswill be apparent to one of ordinary skill in the art in view of thedrawings, specification, and claims. Moreover, it should be noted thatthe language used in the specification has been principally selected forreadability and instructional purposes, and may not have been selectedto delineate or circumscribe the inventive subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the embodiments of the present invention can be readilyunderstood by considering the following detailed description inconjunction with the accompanying drawings.

FIGS. 1A-1D illustrate various embodiments of a switching powerconverter.

FIGS. 2A-2B illustrate example embodiments of a detection circuit.

FIGS. 3A-3B are example waveforms illustrating signals in the detectioncircuit during a voltage ringing period.

FIG. 4 illustrates example waveforms corresponding to a switching powerconverter during light- or no-load operation.

FIG. 5 illustrates example waveforms corresponding to a switching powerconverter with an applied load transient.

FIG. 6 illustrates another example embodiment of a detection circuit.

FIG. 7 illustrates example waveforms corresponding to a switching powerconverter with an applied load transient.

DETAILED DESCRIPTION OF EMBODIMENTS

The figures and the following description relate to preferredembodiments of the present invention by way of illustration only. Itshould be noted that from the following discussion, alternativeembodiments of the structures and methods disclosed herein will bereadily recognized as viable alternatives that may be employed withoutdeparting from the principles of the claimed invention.

Reference will now be made in detail to several embodiments of thepresent invention(s), examples of which are illustrated in theaccompanying figures. It is noted that wherever practicable similar orlike reference numbers may be used in the figures and may indicatesimilar or like functionality. The figures depict embodiments of thepresent invention for purposes of illustration only. One skilled in theart will readily recognize from the following description thatalternative embodiments of the structures and methods illustrated hereinmay be employed without departing from the principles of the inventiondescribed herein.

Embodiments of switching power converters described herein may beconfigured to (1) detect a dynamic load at an output of the switchingpower converter using a secondary-side detection device, and (2) signala primary-side controller of the occurrence of a dynamic load, withoutimpacting the feedback or regulation control loop, thus not impactingloop stability, and without increasing no-load power consumption. Thecontroller regulates the output voltage by modifying the on and offtimes of the power switch based on the signal indicating a dynamic load.

FIGS. 1A-1D illustrate example embodiments of a switching powerconverter 100. In one embodiment, the switching power converter 100includes, among other components, a transformer with primary winding102, bias winding 103, and secondary winding 104, a power switch 106,and a controller 110.

Referring to FIGS. 1A-1D, the power converter 100 receives AC power froman AC power source (not shown), which is rectified to provide theregulated DC input voltage 101 across the input capacitor C1. Inputvoltage 101 is coupled to the primary winding 102. During

ON cycles of the power switch 106, energy is stored in the primarywinding 102 because the rectifier D₁ is reverse biased. The energystored in the primary winding 102 is released to the secondary winding104 and transferred to the load 120 across the capacitor C2 during theOFF cycles of the power switch 106 because the rectifier D₁ becomesforward biased. After the power switch 106 turns off, the rectifier D1conducts current to the output of the switching power converter 100. Atthe end of the conduction period, the voltage across the rectifier D1resonates due to the inductance and parasitic capacitance of thetransformer. The rectifier D₁, which comprises, for example, a diode ora synchronous rectifier, rectifies the voltage of the secondary winding104, and the capacitor C2 filters the voltage of the secondary winding104 for outputting as output voltage 121 across load 120.

The primary-side controller 110 generates a control signal 113 to turnon or turn off power switch 106. The controller 110 senses current Isense through the primary winding 102 in the form of a voltage 115across a sense resistor Rs. The current I sense is proportional to thecurrent through the load 120 by a turns ratio of the transformer. Thecontroller 110 also receives a sensed feedback voltage V_senseindicative of the output voltage 121, which may be generated in avariety of manners. In one embodiment, the voltage across the secondarywinding 104 is reflected across a bias winding 103 of the transformerduring OFF cycles of the power switch 106. In this case, the voltageacross the bias winding 103 is divided by a resistor divider and inputto the controller 110 as the sensed voltage V sense. Any of a variety ofother feedback mechanisms may be used to sense the output voltage 121 asfeedback to the controller 110. The controller 110 controls switching ofthe power switch 106 to regulate the output voltage 121 based on V_senseor to regulate output current through the load 120 based on I sense. Thecontroller 110 can employ any one of a number of modulation techniques,such as pulse-width-modulation (PWM) or pulse-frequency-modulation(PFM), to control the ON and OFF states and duty cycles of the powerswitch 106 to regulate the output voltage 121 and current through theload 120.

The controller 110 is configured to operate the switching powerconverter 100 during a variety of load conditions, including when a load(e.g., an electronic device) is connected to the power supply and when aload is not connected. For example, in a constant voltage mode, thecontroller 110 supplies a regulated DC output of a fixed voltage withina specified tolerance range. Constant voltage mode generally indicatesthat the internal battery of the electronic device is fully charged andthe fixed voltage output of the power supply provides the operatingpower for the electronic device to be operated normally. In a constantcurrent mode, the power supply provides a fixed current output. Constantcurrent mode generally indicates that the internal battery of theelectronic device is not fully charged and the constant current outputof the power supply allows for the efficient charging of the internalbattery of the electronic device. Lastly, in a no-load condition, theelectronic device is disconnected from the power supply. Under theno-load condition, the controller 110 may maintain a regulated voltageoutput from the power converter 100 in anticipation of the load beingreconnected to the power supply.

Under light-load or no-load conditions, controller 110 may operate inPFM and reduce the switching frequency of power switch 106 in order tomaintain regulation of output voltage 121. As the switching frequency ofpower switch 106 decreases, the time between measurements of the sensedvoltage V_sense due to the OFF cycles of the power switch 106 increases.If the load 120 increases between measurements, the output voltage 121drops until V_sense is sensed again and the controller 110 responds tothe load change.

The detection circuit 130 detects changes in the load 120 betweenmeasurements of the feedback voltage in each off cycle of the powerswitch 106. The detection circuit 130 is coupled across the rectifier D1on the secondary side of the transformer and measures a voltage V_RECacross the rectifier. In embodiments of the switching power converter100 having a diode as the rectifier D1, the detection circuit 130includes a first pin coupled to the cathode of the rectifier D1 and asecond pin coupled to the anode of the rectifier D1, and measures avoltage from the cathode to the anode of the rectifier D1. During a loadtransient, the voltage V_REC across the rectifier D1 as measured by thedetection circuit 130 indicates a change in the output voltage 121. Ifthe detection circuit 130 detects a load transient, the detectioncircuit 130 provides a low impedance between its pins, allowing currentto flow through the secondary winding 104. The controller 110 detectsthe change in the sensed voltage V sense as indication of the current inthe secondary winding and controls switching of the power switch 106 torespond to the load transient. The detection circuit 130 thereforenotifies the controller 110 of dynamic load conditions occurring duringoff cycles of the power switch 106, enabling the controller 110 toregulate the output voltage and compensate for the dynamic load beforethe output voltage 121 decreases significantly.

As shown in FIG. 1A, one embodiment of the power converter 100 has therectifier D1 coupled between a secondary-side ground and the secondarywinding 104, such that the detection circuit 130 is coupled to thesecondary-side ground and the secondary winding 104. However, variousalternative configurations of the detection circuit 130 relative toother components of the power converter 100 are also possible. Inparticular, because the detection circuit 130 detects a voltage acrossthe rectifier D1, the detection circuit 130 does not need to be coupledto the secondary-side ground. Accordingly, the position of the rectifierD1 and detection circuit 130 may be varied to improve performance of thepower converter 100 for electromagnetic interference or otherconsiderations.

For example, FIG. 1B illustrates an alternative configuration of aswitching power converter 100 with the detection circuit 130. In theexample of FIG. 1B, the rectifier D1 is coupled between the secondarywinding 104 of the transformer and the output capacitor C2, and thedetection circuit 130 is coupled across the rectifier D1.

FIG. 1C illustrates another alternative configuration of a switchingpower converter 100 with the detection circuit 130. In the example ofFIG. 1C, the rectifier D1 and detection circuit 130 are coupled betweenthe secondary winding 104 and a second secondary winding 410 of thetransformer.

FIG. 1D illustrates yet another configuration of a switching powerconverter 100 with the detection circuit 130. In the example of FIG. 1D,the detection circuit 130 and rectifier (e.g., rectifier D1) areintegrated into a smart diode 140, or single IC chip serving as arectifier while providing dynamic load detection. The smart diode 140 isplaced on a secondary side of the transformer. For example, FIG. 1Dillustrates the smart diode 140 in series with the secondary winding 104of the transformer and the output capacitor C2. However, otherembodiments of the power converter 100 may have different configurationsof the smart diode 140 relative to the secondary-side components of thetransformer.

FIGS. 2A-2B are block diagrams illustrating example embodiments of thedetection circuit 130. In one embodiment, as shown in FIG. 2A, thedetection circuit 130 comprises a comparator 210, a blanking circuit220, and a pulse generator 230. The detection circuit 130 has a firstpin STR coupled to one side of the rectifier and a second pin GNDcoupled to the other side of the rectifier. Other embodiments of thedetection circuit 130 may include additional or different components.

The pulse generator 230 turns on and turns off a bypass switch 235 basedon the pulse enable signal PL_EN. If the pulse enable signal PL_EN islow, the pulse generator 230 is deactivated and does not turn on thebypass switch 235. In this state, the impedance between the pins of thedetection circuit 130 is high. If the pulse enable signal PL_EN is high,the pulse generator 230 generates a control signal to turn on the bypassswitch 235 for one or more short-duration pulses (e.g., each pulse 500ns in duration). When the bypass switch 235 is turned on, the impedancebetween the pins STR and GND of the detection circuit 130 is equivalentto the resistor R1 and any internal resistance in the bypass switch 235and wire. In one embodiment, the resistor R1 has a relatively smallresistance, resulting in a low impedance between the two pins of thedetection circuit 130 while the bypass switch 235 is turned on. Thelow-impedance path provided by the bypass switch 235 being turned onenables current to bypass the rectifier D1, generating a current in thesecondary winding 104. The bypass switch 235 may be a MOSFET in someembodiments, as shown for example in FIG. 2A, or may be any of a varietyof other types of switches.

The pulse enable signal PL_EN is generated based on an output of thecomparator 210 and the blanking circuit 220. The voltage between thefirst and second pins STR and GND of the detection circuit 130 isdivided by a resistor divider 215, and the divided voltage is input tothe comparator 210. The comparator 210 compares the divided voltage to athreshold voltage (e.g., 1.25V) and outputs a binary high value if thedivided voltage falls below the threshold voltage. The threshold voltageis, for example, set by a manufacturer of the load 120 as a lowerboundary on the voltage specifications for the load 120.

The blanking circuit 220 outputs a binary high or low value torespectively enable or disable the pulse generator 230. In particular,as turning on the bypass switch 235 during an on pulse of theprimary-side power switch 206 would allow current to bypass therectifier D1 and therefore coupled the input and output capacitors C1and C1, the blanking circuit 220 outputs a binary low value to disablethe pulse generator 230 when the power switch 106 is turned on.Furthermore, to reduce false triggers of the detection circuit 130, theblanking circuit 220 may also disable the pulse generator 230 during atleast a portion of a resonance period in which the transformerinductance and parasitic capacitance resonate. The blanking circuit 220outputs a binary high value within a specified amount of time after thepower switch 106 turns on, enabling the detection circuit 130 to detectto a change in the voltage across its pins until the power switch 106turns on again.

In one embodiment, the blanking circuit 220 includes a comparator and adelay timer. The comparator detects either a rising edge of the voltagebetween the first and second pins of the detection circuit 130 or afalling edge of the voltage between the first and second pins as thevoltage across the detection circuit 130 increases above a thresholdvoltage (e.g., output voltage 121) or falls below the threshold voltage,and activates the delay timer in response. The delay timer expires aftera predefined amount of time. The output of the blanking circuit 220 islow before the delay timer expires, setting the pulse enable signalPL_EN to a low value. The output of the blanking circuit 220 is highafter the delay timer expires, setting the pulse enable signal PL_EN toa high value if the output of the comparator 210 is also high.

Another embodiment of the blanking circuit 220 includes two comparators.The first comparator detects the time the voltage across the detectioncircuit 130 rises above an upper threshold voltage and outputs a highvalue while the voltage across the detection circuit 130 is above theupper threshold. The second comparator detects a time the operatingvoltage VCC falls below a lower threshold voltage and outputs a highvalue while VCC is below the lower threshold.

If the outputs of the blanking circuit 220 and the comparator 210 areboth a binary high value, the pulse enable signal PL_EN becomes a binaryhigh value. That is, if the voltage between the first and second pins ofthe detection circuit 130 falls below a threshold voltage outside of aspecified time period after the power switch 106 turns on, the pulseenable signal PL_EN is high. Otherwise, the pulse enable signal PL_EN islow.

As the ringing period of the transformer inductance and parasiticcapacitance may vary significantly, the time period over which theblanking circuit 220 deactivates the detection circuit 130 may also varysignificantly. A shorter blanking period enables the detection circuit130 to detect dynamic loads occurring during a larger portion of theswitching cycle. However, if the blanking period is too short, theringing may cause a false trigger of the detection circuit 130. Toprovide fast yet reliable detection of dynamic load conditions, anotherembodiment of the detection circuit 130, shown in FIG. 2B, includes ade-glitch filter 240 coupled between the comparator 210 and the pulsegenerator 230. In general, the de-glitch filter 240 reduces the effectof transformer ringing on the detectability of a change in the outputvoltage 121. The de-glitch filter 240 filters out pulses in a signalCOMP DET received from the comparator 210 that have an above-thresholdmagnitude and a below-threshold width. The de-glitch filter 240generates a filtered signal COMP_DET_DG. The pulse enable signal PL_ENis high if the output of the blanking circuit 220 and the filteredsignal COMP_DET_DG are high. The pulse generator 230 turns on the bypassswitch 235 in response to the high pulse enable signal PL_EN.

A capacitor C3 stores the voltage Vcc for powering the variouscomponents of the detection circuit 130. In the embodiments of FIGS.2A-2B, the capacitor C3 has a large capacitance to hold the Vcc voltageabove an under voltage lockout (UVLO) level of the detection circuit 130during the time the voltage across the detection circuit 130 is zero.Because the Vcc voltage is maintained above the UVLO level, thedetection circuit 130 does not need to complete a startup sequence eachtime the voltage V_REC goes high. Accordingly, the large capacitance ofthe capacitor C3 enables the detection circuit 130 to quickly beginmonitoring the voltage V_REC in each switching cycle of the power switch106.

FIGS. 3A and 3B are example waveforms illustrating signals in thedetection circuit 130 of FIG. 2B during a ringing period of the voltageV_REC. Specifically, FIG. 3A is an example waveform illustrating inputsto the comparator 210. Specifically, the comparator 210 receives areference voltage defining a voltage threshold and the voltage output bythe resistor divider 215. The voltage output by the resistor divider 215oscillates around the reference voltage due to resonance of thetransformer. FIG. 3B is an example waveform illustrating the output ofthe comparator 210 (that is, the input to the de-glitch filter 240) andthe output of the de-glitch filter 240. As shown in FIG. 3B, the outputof the comparator 210 switches between high and low values as the inputof the comparator 210 oscillates around the reference value. Thede-glitch filter 240 filters out the high-frequency oscillations in thecomparator output signal, generating a smooth signal as shown in FIG.3B. Between times A and B shown in FIGS. 3A and 3B, the comparatoroutput oscillates between low and high values. The de-glitch filter 240filters out the high-frequency oscillations and outputs a low valueafter time B.

FIG. 4 illustrates example waveforms of embodiments of switching powerconverter 200 under constant-load conditions. At time t0, the switchdrive signal 113 becomes high, turning on the primary-side power switch106. The voltage V_REC across the secondary-side rectifier D1 increasesin response to the power switch 106 turning on because the rectifier D1becomes reverse biased. The switch drive signal 113 falls to a low valueat time t1 to turn off power switch 106, forward biasing the rectifierD1 and causing the voltage V_REC to fall to a substantially zero value.The rectifier D1 conducts current to the output of the switching powerconverter 100 between times t1 and t2, and the voltage V_REC oscillatesfrom time t2 to time t3 due to transformer leakage inductance andparasitic capacitance. The time t3 represents the time the magnitude ofthe oscillations falls below a threshold. The voltage V_REC remainssubstantially constant between times t3 and t4, at which time the switchdrive signal 113 again becomes high to turn on the power switch 106.Thus, the period between times t0 and t4 represents a switching periodof the transformer. The output voltage 121 across the load remains at asubstantially constant value throughout the switching period. Becausethe output voltage 121 does not significantly change, the detectioncircuit 130 is not activated in the switching cycle shown in FIG. 4.

FIG. 5 illustrates example waveforms of embodiments of the powerconverter 100 during transient load conditions. Similar to FIG. 4, theswitch drive signal 113 is high between times t0 and t1, resulting inthe power switch 106 being on from t0 to t1. The voltage V_REC increasesat the rising edge of the switch drive signal 113 and decreases at thefalling edge of the switch drive signal 113, at time t1. The voltageV_REC oscillates from times t2 to t3, which represents a time themagnitude of the oscillations is greater than a threshold magnitude. Attime t3 _(—) 1, the load driven by the switching power converter 100 isincreased, causing the output voltage 121 to begin falling. As theoutput voltage 121 decreases, the voltage V_REC also decreases until itfalls below a comparator threshold 504 at time t3_2 (indicated by ahorizontal dotted line in FIG. 5). In response to the voltage V_RECfalling below the threshold, the detection circuit 130 activates thebypass switch 235, creating a low-impedance path between the two pins ofthe detection circuit 130. The low-impedance path enables current tobypass the rectifier D1, creating voltage spikes on the transformerswindings and a current spike at the STR pin of the detection circuit130. The controller 110 detects the voltage spikes on the primary sideof the transformer. For example, the controller 110 detects voltagespikes across the primary winding 102 or the bias winding 103 of thetransformer. In response to detecting the voltage spike, the controller110 activates the power switch 106. Thus, the power switch 106 turns onat time t4.

In one embodiment, as shown in FIG. 5, the blanking circuit 220deactivates the detection circuit 230 for a blanking period 502, or aperiod of time after t0. That is, the output V_blanking of the blankingcircuit 220 is high during the blanking period 502 and is low outside ofthe blanking period 502. Accordingly, the bypass switch 235 of thedetection circuit 130 is not activated during the blanking period 502and the detection circuit 130 does not alert the controller 110 tochanges in the output voltage 121. In one embodiment, a voltage Vundershoot en is also low during the transformer reset time (between t1and t2 in FIG. 5) to disable detection of undershoot of the outputvoltage Vout 121. The blanking period 502 includes the time the powerswitch 106 is on, as enabling the bypass switch 235 of the detectioncircuit 130 during the on-time of the power switch 106 would couple theinput capacitor C1 to the output capacitor C2 and thereby generate alarge current in the power converter 100. In one embodiment, theblanking period 502 also includes the time the secondary rectifierconducts current, as shown in FIG. 4 from t1 to t2, as the secondaryrectifier voltage is close to zero which may falsely trigger thecomparator 210. In one embodiment, the blanking period 502 also includesa portion of each off cycle of the power switch 106. For example, FIG. 5illustrates the blanking period 502 extending from t0 (when the powerswitch 106 turns on) until t3 (when oscillations in the voltage V_RECfall below a threshold magnitude). The blanking period 502 may beshorter or longer in other embodiments. For example, in embodiments ofthe detection circuit 130 comprising a de-glitch filter 240 tocompensate for the high-frequency oscillations between times t2 and t3,the blanking circuit 220 may output a low value from times t0 to t2 andotherwise output a high value. In this case, the blanking period 502 isthe time between times t0 and t2.

In the embodiment illustrated in FIG. 5, the capacitor C3 of thedetection circuit 130 has a relatively large capacitance, such that theoperating voltage VCC does not fall below the UVLO level of thedetection circuit 130 during the time V_REC is zero.

FIG. 6 is a block diagram illustrating another embodiment of thedetection circuit 130. In the embodiment of FIG. 6, a fastpower-on-reset (POR) device 605 is coupled to the detection circuit 130.The fast POR device 605 is configured to quickly start up the componentsof the detection circuit 130 between switching cycles of the powerswitch 106. For example, the fast POR device 605 starts up thecomponents of the detection circuit 130 within several microseconds ofthe voltage V_REC going high. In one embodiment, the capacitor C3 has alower capacitance than in the embodiments of FIGS. 2A and 2B, reducingthe size of the capacitor C3 and thereby reducing the overall size ofthe detection circuit 130.

FIG. 7 illustrates example waveforms of the embodiment of the detectioncircuit 130 shown in FIG. 6. Similar to FIG. 5, the switch drive signal113 is high between times t0 and t1, resulting in the power switch 106being on from t0 to t1. The voltage V_REC increases at the rising edgeof the switch drive signal 113 and decreases at the falling edge of theswitch drive signal 113, at time t1. The voltage V_REC oscillates fromtimes t2 to t3, and begins falling at time t3_1 in response to the loaddriven by the switching power converter 100 being increased. V_REC fallsbelow the comparator threshold 504 at time t3 2, and the detectioncircuit 130 activates the bypass switch 235 in response.

Because the capacitor C3 of the embodiment of the detection circuit 130associated with FIG. 7 has a relatively low capacitance, the operatingvoltage VCC begins decreasing at time t1 when the voltage V_REC falls tozero. At time t1′, VCC falls below the UVLO level of the detectioncircuit 130, disabling the detection circuit 130. Furthermore, as shownin FIG. 7, the voltage V_blanking output by the blanking circuit 220 ishigh from time t0 to time t1′. For example, the high output is triggeredin response to the rising edge of the voltage V_REC. The voltageV_blanking returns to low when the operating voltage VCC falls below theUVLO level. When the voltage V_REC begins oscillating at time t2 (thatis, when V_REC is greater than zero), the capacitor C3 begins chargingand the operating voltage VCC increases. The fast POR device 605generates a startup signal to activate the components of the detectioncircuit 130 after the voltage VCC increases above the UVLO level. Forexample, the fast POR device 605 outputs a high startup ready signalwhen the detection circuit 130 is ready to detect changes in the voltageV_REC. While the startup ready signal is high, the detection circuit 130is configured to detect decreases in the voltage V_REC and generatepulses detectable by the primary-side controller 110.

While particular embodiments and applications have been illustrated anddescribed herein, it is to be understood that the embodiments are notlimited to the precise construction and components disclosed herein andthat various modifications, changes, and variations may be made in thearrangement, operation, and details of the methods and apparatuses ofthe embodiments without departing from the spirit and scope of theembodiments as defined in the appended claims.

Upon reading this disclosure, those of skill in the art will appreciatestill additional alternative designs for the system. Thus, whileparticular embodiments and applications of the present invention havebeen illustrated and described, it is to be understood that theinvention is not limited to the precise construction and componentsdisclosed herein and that various modifications, changes and variationswhich will be apparent to those skilled in the art may be made in thearrangement, operation and details of the method and apparatus of thepresent invention disclosed herein without departing from the spirit andscope of the invention as defined in any claims drawn to the subjectmatter herein.

What is claimed is:
 1. A switching power converter comprising: atransformer including a primary winding coupled to an input voltage anda secondary winding coupled to an output of the switching powerconverter; a power switch coupled to the primary winding of thetransformer, current in the primary winding being generated responsiveto the power switch being turned on and not generated responsive to thepower switch being turned off; a rectifier coupled to the secondarywinding of the transformer, the rectifier conducting a rectified currentto the output of the switching power converter during off cycles of thepower switch; a detection circuit measuring voltage across therectifier, the detection circuit detecting a decrease in the voltageacross the rectifier outside of a blanking period and generating one ormore current pulses in the secondary winding of the transformer inresponse to detecting the voltage decrease; and a fast power-on-reset(POR) device coupled to the detection circuit, the fast POR devicegenerating a startup signal to activate the detection circuit at an endof the blanking period.
 2. The switching power converter of claim 1,wherein the detection circuit comprises: two pins coupled to respectivesides of the rectifier; and a bypass switch, an impedance between thetwo pins being high responsive to the bypass switch being turned off andlow responsive to the bypass switch being turned on.
 3. The switchingpower converter of claim 2, further comprising a pulse generator coupledto the bypass switch, the pulse generator generating a control signal toturn on the bypass switch in responsive to a voltage between the twopins falling below a threshold voltage.
 4. The switching power converterof claim 2, wherein the detection circuit comprises a blanking circuitturning off the bypass switch during the blanking period.
 5. Theswitching power converter of claim 4, wherein the blanking circuitcomprises: a comparator detecting an on-time of the power switch; and adelay timer adapted to turn on in response to the detected on-time ofthe power switch and turn off after a preset amount of time, wherein theblanking circuit turns off the bypass switch in response to the delaytimer being turned on.
 6. The switching power converter of claim 5,wherein the detection circuit comprises a de-glitch filter adapted tofilter out a high frequency ringing of a voltage output by thecomparator.
 7. The switching power converter of claim 1, wherein theblanking period includes an on-time of the power switch.
 8. Theswitching power converter of claim 7, wherein the blanking periodfurther includes a portion of an off-time of the power switch while therectifier conducts current to the output of the switching powerconverter.
 9. The switching power converter of claim 1, wherein thevoltage across the rectifier rings after the rectifier conducts currentto the output of the switching power converter, and wherein the blankingperiod includes at least a portion of a time the voltage across therectifier is ringing.
 10. The switching power converter of claim 1,further comprising: a switch controller generating a control signal toturn on and turn off the power switch, the switch controller adapted todetect the current pulse generated by the detection circuit as afeedback voltage across one of the primary winding and a bias winding ofthe transformer, the switch controller adapted to turn on the powerswitch in response to detecting the current pulse.
 11. A method forcontrolling a switching power converter, the switching power converterincluding a transformer with a primary winding coupled to an inputvoltage and a secondary winding coupled to an output of the switchingpower converter, a power switch coupled to the primary winding of thetransformer, a rectifier coupled to the secondary winding of thetransformer, and a detection circuit measuring voltage across therectifier, the method comprising: generating a startup signal toactivate the detection circuit at an end of a blanking period; detectinga decrease in a voltage across the rectifier outside of the blankingperiod; and responsive to detecting the voltage decrease, generating oneor more current pulses in the secondary winding of the transformer. 12.The method of claim 11, further comprising: detecting the voltage acrossthe rectifier by two pins coupled to respective sides of the rectifier;wherein generating the one or more current pulses comprises turning on abypass switch, the bypass switch when on providing a low impedance pathbetween the two pins.
 13. The method of claim 12, further comprisinggenerating a control signal to turn on the bypass switch in responsiveto the voltage between the two pins falling below a threshold voltage.14. The method of claim 12, further comprising turning off the bypassswitch during the blanking period.
 15. The method of claim 14, whereinturning off the bypass switch during the blanking period comprises:detecting an on-time of the power switch; and responsive to detectingthe on-time of the power switch, turning on a delay timer having apreset expiration time, wherein the bypass switch is turned off inresponse to the delay timer being turned on.
 16. The method of claim 11,further comprising applying a de-glitch filter to an output of acomparator, the comparator detecting the voltage drop across therectifier, the de-glitch filter filtering out high frequency pulses inthe output of the comparator caused by a ringing of the voltage acrossthe rectifier.
 17. The method of claim 11, wherein the blanking periodincludes an on-time of the power switch and a portion of an off-time ofthe power switch while the rectifier conducts current to the output ofthe switching power converter.
 18. The method of claim 11, wherein thevoltage across the rectifier rings in response to the secondaryrectifier stops conduct current, and wherein the blanking periodincludes at least a portion of a time the voltage across the rectifieris ringing.
 19. The method of claim 11, wherein the switching powerconverter further comprises a switch controller generating a controlsignal to turn on and turn off the power switch, the method furthercomprising: detecting the current pulse generated by the detectioncircuit by monitoring a voltage across one of the primary winding and abias winding of the transformer; and turning on the power switch inresponse to detecting the current pulse.
 20. A switching powerconverter, comprising: a transformer including a primary winding coupledto an input voltage and a secondary winding coupled to an output of theswitching power converter; a power switch coupled to the primary windingof the transformer, current in the primary winding being generatedresponsive to the power switch being turned on and not generatedresponsive to the power switch being turned off; and a smart diodecoupled to the secondary winding of the transformer, the smart diodecomprising: a rectifier conducting a rectified current to the output ofthe switching power converter during off cycles of the power switch; adetection circuit measuring voltage across the rectifier, the detectioncircuit detecting a decrease in the voltage across the rectifier outsideof a blanking period and generating one or more current pulses in thesecondary winding of the transformer in response to detecting thevoltage decrease; and a fast power-on-reset (POR) device coupled to thedetection circuit, the fast POR device generating a startup signal toactivate the detection circuit at an end of the blanking period.